Thermal sensor with increased sensitivity

ABSTRACT

The invention provides a thermal sensor having a first and second temperature sensing elements each being formed on a thermally isolated table in a first substrate.

RELATED APPLICATIONS

This application is a continuation-in-part of co-pending U.S. patentapplication Ser. No. 11/045,910, filed Jan. 26, 2005 and titled“Sensor,” and is a continuation of International ApplicationPCT/EP/050174, filed Jan. 12, 2006, the latter claiming Paris Conventionpriority to said U.S. patent application Ser. No. 11/045,910, whichapplications are hereby incorporated by reference.

FIELD OF THE INVENTION

The present invention relates to sensors and in particular to a thermalsensor with increased sensitivity. The invention more particularlyrelates to a thermal sensor with a first and second temperature sensingelements, typically resistive elements, co-located on a thermallyisolated table, such that each of the first and second temperaturesensing elements are exposed to the same thermal environment.

BACKGROUND

Sensors are well known in the art. When formed in a semiconductormaterial such as silicon or germanium such sensors may be provided asmechanical structures, for example as a MEMS arrangement, orelectro-magnetic (EM) radiation sensors such as infra-red (IR) sensors.By using materials such as silicon it is possible to form the sensor inone or more layers of the wafer from etching and other semiconductorprocessing techniques so as to result in a desired configuration. Due tothe delicate nature of the sensors and their sensitivity to thesurrounding environment it is known to provide a protective cap over thesensor, the cap serving to isolate the environment of the sensor fromthe ambient environment where the sensor is operable.

Within the area of EM sensors there is a specific need for sensors thatcan be provided in a packaged form.

SUMMARY

These and other problems are addressed by an electrical circuitproviding thermal sensor in accordance with the teaching of theinvention which is provided with first and second temperature sensingelements, typically resistive elements, co-located on a thermallyisolated table, such that each of the first and second temperaturesensing elements are exposed to the same thermal environment. Byco-locating the sensing elements in an isothermal environment it ispossible to ensure that the response from the sensing elements is aresponse related to the irradiation thereon as opposed to beingderivable from some other source.

In accordance with preferred embodiments, the invention thereforeprovides an electrical circuit according to claim 1. Advantageousembodiments of such an electrical circuit are provided in dependentclaims thereto. The invention also provides a thermal sensor accordingto claims 17 or 18, a sensor array according to claim 62, a gas analyseraccording to claim 66 and a discriminatory sensor according to claim 64.The invention also provides a method of forming a sensor according toclaim 68. An electromagnetic sensor according to the teaching of claim70 is also provided.

These and other features of the teaching of the invention will beunderstood with reference to the following drawings which are providedas an aid to understand the teaching as opposed to being construed inany way limiting to the scope.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described with reference to theaccompanying drawings in which:

FIG. 1 is a cross section through an illustrative embodiment of a sensorfor practicing the present invention.

FIG. 2 is a perspective view from above of the sensor of FIG. 1.

FIG. 3 is an example of a methodology that may be employed for formingthe sensor of FIG. 1.

FIG. 4A is an example of a first pattern that may be used to define anoptical element in accordance with the teachings of the presentinvention.

FIG. 4B is an example of a second pattern that may be used to define anoptical element in accordance with the teachings of the presentinvention.

FIG. 4C is an example of a third pattern that may be used to define anoptical element in accordance with the teachings of the presentinvention.

FIG. 5 is a plan schematic showing an example of a sensor includingmultiple sensor elements in accordance with an illustrative embodimentof the invention.

FIG. 6 is an example of a pattern that may be used to define an opticalelement suitable for use with multiple sensor elements in FIG. 5 inaccordance with the teachings of the present invention.

FIG. 7 is a sectional view of a compound sensor in accordance with theteachings of the invention.

FIG. 8 shows a further embodiment where the sensor includes a referenceelement.

FIG. 9 shows a modification to the arrangement of FIG. 8.

FIG. 10 shows an exemplary embodiment of a sensor configuration that maybe used within the context of the present invention.

FIG. 11 shows in the context of a further embodiment the provision ofsensor elements on a thermally insulated table.

FIG. 12 shows in the context of a further embodiment the formation of athermal barrier between a sensor and surrounding elements on thesubstrate, 12 a being a section view and 12 b a plan view of thearrangement.

FIG. 12 c shows a modification to the arrangement of FIGS. 12 a & 12 b.

FIG. 13 shows a further modification to the arrangement of FIG. 12.

FIG. 14 shows in the context of a further embodiment the provision ofdie temperature sensors.

FIG. 15 shows an example of an array of the die temperature sensors ofFIG. 14.

FIG. 16 shows a further example of an array of the die temperaturesensors.

FIG. 17 shows a further example of an array of the die temperaturesensors.

DETAILED DESCRIPTION OF THE DRAWINGS

The invention will now be described with reference to exemplaryembodiments of FIGS. 1 to 17. Although the invention has application inany electro-magnetic (EM) radiation sensing environment, for the ease ofexplanation it will now be described with reference to a preferredillustrative embodiment, that of a silicon wafer-based thermal radiationsensor. While it is possible for each of the embodiments illustratedhereinafter to be used in combination with one another it will beunderstood that the invention is not to be construed in this limitingfashion as features and components of one embodiment may or may not beused with those of another embodiment. In this way the invention is onlyto be limited insofar as deemed necessary in the light of the appendedclaims.

Electromagnetic radiation sensors often contain delicate sensingmembranes. The fragile nature of the membrane necessitates careful (withresultant cost repercussions) handling of the sensor after the membranehas been manufactured to prevent damage and yield loss. In addition, formembrane-based thermal radiation sensors, it is an advantage to packagethe sensor in a vacuum or other low pressure environment to eliminateheat loss from the absorbing membrane through gas convection andconduction. Finally, while many single point IR sensors do not use afocusing lens at all, it is an advantage in single point thermal sensorsto be able to focus the incoming radiation onto a single sensitive pointon the membrane to effectively amplify the signal. In the cases wheresingle point IR sensors are using a lens, they generally use arefractive lens of a material with a suitable shape and refractiveindex, for example germanium or other similar material.

For imaging a thermal scene onto a sensor array to produce an infraredpicture of the scene, the same requirements also apply with theadditional requirement that focusing the beam (i.e. with a lens) ishighly desirable to produce a focused image of the scene on the imageplane of a sensor array.

The sensor of the present invention addresses these and other challengesdescribed above by providing a device and method for capping the thermalsensor at the wafer level with a silicon cap. In accordance with thepresent invention a sensor device (or array of repeating sensor devices)is manufactured on one wafer substrate and a capping wafer ismanufactured on a separate substrate. The capping wafer is joined to thesensor wafer and bonded to it under controlled ambient conditions, thepreferred embodiment being under vacuum conditions. This bonded waferarrangement can be singulated or sawn into individual capped sensorchips for final packaging and sale. Such capping methodologies are welldescribed in US Application No. 20030075794 of Felton et al which isassigned to the Assignee of the present invention, and the contents ofwhich are incorporated herein by reference.

FIG. 1 shows in cross section, such a sensor device 100. The deviceincludes a sensing element 105 formed in a first silicon wafer 110 orwhat is sometimes called a sensor die. A cap 115 consisting of a siliconlid into which patterns 120 are etched to form an individual diffractingoptical element is also provided. Two possible approaches toimplementing this diffractive optical element (DOE) are known asamplitude modulation and phase modulation respectively. In the case ofamplitude modulation, the surface pattern consists of areas that allowtransmission of the radiation and areas that block the radiation. In thecase of phase modulation the pattern consists of height variations onthe surface that effectively modify the relative phase of the radiationas a function of the relative height differences of the pattern. In thisillustrated embodiment the pattern is provided on an interior surface135 of the cap, but it will be appreciated that it could also beprovided on an exterior surface 140. It will also be appreciated thatthe pattern, whose geometry is exaggerated for ease of viewing, includesa plurality of ridges 150 whose distance apart and depth is related tothe wavelength of light with which the optical element is being used.The cap is typically formed in a second silicon wafer or capping die.This pattern 120 defined in the diffracting optical element cap 115 iscapable of focusing incident radiation 125 of a given frequency onto aspecific plane of the sensor or onto a specific point on the sensor orof focusing different frequencies onto different points. The cap 115 isbonded to the first wafer using a bond or seal material 130 and thebonding defines a sealed cavity 145, which can be at a differentpressure than ambient pressure, typically a lower pressure.Alternatively the sealed nature of this cavity and the manufacturingprocess allows the ambient gas within the cavity to be different to air,for example we could use Xenon which has a lower thermal conductivitythan air or some other gas. Although a silicon cap is substantiallyopaque to incident light in the visible spectrum and therefore it may beconsidered that it occludes the light from impinging on the sensingelement within, it will be appreciated that silicon allows atransmission of light in the infra-red frequencies of the EM spectrumand therefore for this application, the provision of an IR sensor, it isa suitable material. FIG. 2 shows an example of an assembled sensordevice from which it will be seen that the sensing element is covered bythe cap provided above it.

A typical process flow for manufacture of the sensor is shown in FIG. 3.Firstly, the sensor wafer 110 is manufactured using techniques that willbe well known to those in the art (Step 300). The capping wafer is alsomanufactured (Step 310) separately. The manufacture of this cappingwafer includes the etching of a desired pattern on either or both of theouter 140 or inner surface 135 of the cap. An anti-reflective coatingmay additionally be added to the cap surface, either inner or outer.Once the desired components on each of the two wafer substrates areprovided, the wafers may be brought together so as to be bonded (Step320). Ideally, this bonding is achieved under vacuum conditions. Oncethe two wafers have been brought together individual chips may besingulated or defined within the total area of the wafers by removingthe areas of the second wafer that do not define the cap (Step 330). Inthis manner a plurality of individual chips or sensors may be providedin one process flow.

It will be understood that the nature of the pattern defining theoptical element will effect how the sensor performs. FIG. 4 showsexamples of pattern types, which can be implemented using either anamplitude modulation or a phase modulation approach, which may be usedto define diffractive optics in the sensor cap. The example of FIG. 4Ais optimised for a focusing of parallel input light of wavelength 10micrometer down to a focal plane 300 micrometer away using a sinusoidalvariation in the height of the diffractive optical element for a phasemodulation approach. The relative heights of the sinusoid arerepresented by the gray scale variation in the pattern, for an amplitudemodulation approach the gray scale would represent the transmissionefficiency of the pattern. The example of FIG. 4B is designed for afocusing of parallel input light of wavelength 10 micrometer down to afocal plane 370 micrometer away but in this case the black and whitepattern represents a single step height variation to implement thegrating of the phase modulated diffractive optical element rather than asinusoidal variation. The example in FIG. 4C also uses a single stepheight variation to implement the diffractive optical element but inthis case it is designed to focus parallel input light of wavelength 10μm down to a focal plane 10 micrometer away. It will be understood thatthese three examples are illustrative of the type of pattern that may beused and that different design requirements regarding the control of thefocus plane or independent control over different wavelength componentswithin the incident radiation are also possible with this approach andare covered by this invention. These examples, consisting of black andwhite circles in FIGS. 4B and 4C can represent either a transmissionpattern or a phase modulation pattern that focuses the light, but sufferin that losses in transmission are also achieved. It will be appreciatedhowever that the design of the pattern may be optimised to achieve lowerloss criteria such as for example introducing curved side walls in theridge features defining the grating, as represented by the grayscalediagram of FIG. 4A.

The cap provided by the present invention is advantageous in a number ofaspects. It serves to: 1) protect the membrane during subsequenthandling, 2) it also provides a housing for the sensing membrane thatcan be evacuated during manufacture, and 3) it can be patterned andetched in such a way as to focus the incident infra red radiation onto asingle point to amplify the signal or onto an array to create an imageof a scene. In particular, the pattern can be such as to implement anoptical element (i.e. conventional refractive or Fresnel lens) or in thepreferred embodiment a diffractive optical element. The creation of anoptical element for this application is advantageous in that the lenscan be implemented in silicon rather than the more exotic (andexpensive) materials required heretofore for an infrared refractivelens. The advantage resulting from the use of diffractive optics in thesilicon cap is that the lenses can be patterned and etched at the waferbatch level using well established processes and bonded to the sensorwafers, resulting in a cost effective lens compared to the refractivelens technologies heretofore employed. This approach may be applicableto other electromagnetic radiation sensors in addition to the infraredapplication described here. For example the cap could be made of quartzor in some cases standard glasses such as pyrex or possibly sapphire ifthe sensor is to be used for applications other than IR sensors.

In some applications it may also be useful to be able to use thelens/cap configuration to focus different wavelengths within theincoming radiation onto different sensors enclosed by the cap. FIG. 5 isa schematic illustration of one such example where four sensing elements501, 502, 503, 504 are provided within the same cap arrangement. It willbe appreciated that suitable designing of the lens arrangement may allowfor an optimisation of the sensor to focus one particular wavelengthwhile defocusing (rejecting) others. This would allow individualintensity measurement of different wavelength components within theinfrared radiation, a capability that could be very useful in forexample gas analysis such as alcohol breath samplers where there is adesire to monitor the level of ethyl alcohol in the breath of a person.As alcohol has specific absorbance peaks in the IR spectrum, thefocusing of radiation coincident with these peaks onto specific ones ofthe sensors elements 501, 502, 503, 504 provided in an array below thecap will enable the discrimination of any change in the intensity of theradiation at those specific frequencies therefore serve as an indicatorof alcohol present in a sample. As each of the sensor elements areconfigured to react to incident radiation of a suitable frequency, whenthat radiation is incident on the individual sensors, an analysis of theperformance of each of the sensor elements indicates the presence orabsence of the material for which it is designed to react to—providing agas wavelength signature of the gas being analysed.

FIG. 6 is an example of a diffractive optical element (DOE) design usingan amplitude modulation approach that could be used in combination withthe sensor arrangement in FIG. 5 to focus each one of four distinctwavelengths within the incident radiation onto one of the four sensingelements 501, 502, 503, 504 that are shown in FIG. 5. Such a design orpattern could be fabricated by creating a single step in the lens orproviding multiple steps of different heights. It will be appreciatedthat the invention is not intended to be limited in any way as to thefabrication of a DOE in that it is intended to encompass all methods ofmanufacture be they single step, multiple step or other variants.

Although not shown, it will be appreciated that the structure of thepresent invention may be further modified to include a second lensarrangement provided over the optical element so as to effect a compoundlens effect. Such an arrangement may be suitable for applications suchas increasing magnification, increasing the field of view, increasedresolution and improved optical filtering. Such an arrangement could beprovided by providing a second lens coupled to the chip. Alternatively,and as shown in FIG. 7, it is possible to fabricate a second lens 701and couple that second lens to the completed package 700. In this way adefined volume 703 is created between the DOE 115 and the interiorportion of the second lens 701. The atmosphere in the defined volume maybe controlled as desired vis a vis pressure or content. It will beappreciated that any other method of fabricating a compound lens effectis intended to be encompassed within the scope of the invention.

It will be understood that the techniques of the present inventionprovide an efficient way to provide an IR sensor array such as forexample a 60×60 array. Such configurations are desirable forapplications such as IR imaging where a sensor array of the presentinvention may be used to replace conventional IR arrays. Current IRarrays do not have the lens and sensor array integrated in a low costunit as provided for by this invention. Current conventional IR arraysprovide a vacuum package with an IR transparent window or lens in thepackage rather than the wafer level solution described by thisinvention.

Another application for the integrated sensor element/lens capconfiguration of the present invention is where depth of field analysisis required. By configuring the lens suitably, it is possible to focuslight from two different distances onto separate sensor elements withinthe cap. This enables discrimination as to the origin of the heatsource, for example is it a planar metal plate or a 3-Dimensional humantorso. Such applications may include discriminatory deployment sensorsfor use in for example air bag deployment arrangements.

The dimensions of a sensor in accordance with the present invention aretypically of the order of micro to millimetres. For example whentargeting radiation of a wavelength of 10 micrometers, a cap may bedimensioned to have a collection area of about 1 mm² and be of a heightof about 160 micrometers above the sensor element. These dimensions arehowever purely for illustrative purposes only and it is not intended tolimit the present invention to any one set of dimension criteria.

The fabrication of the sensor of the present invention has beendescribed with reference to an etch process. Typically this etch will beof the type of process known as deep reactive ion etching (RIE) whichinherently produces substantially vertical sidewalls (approximately 90degrees). One of the advantages of such a process is that with suchverticality less space is required for the cavity sidewalls. Thisdirectly affects the size of the “window” and thus the overall size ofthe cap which can be made. By reducing the cap size there is a reductionin the area required on the chip-with a corresponding reduction in the“wasted” space under and around the cap edges.

Cap arrangement incorporating a radiation barrier.

Heretofore, a sensor in accordance with the teaching of the inventionhas been described with reference to a sensing device with a transparentwindow. The invention also provides in certain embodiments for thefabrication of a second cell also incorporating a sensing device, whichprovides a different response to that of the first cell. This secondcell then may be considered a reference cell, which differs from thefirst sensing cell in that its response may be used in combination withthe sensing cell to allow for a discrimination in the response of thesensing cell. One example of this is to make the reference cell totallyopaque so its sensor sees only the cap (i.e. 300K) in the case of IRsensors, but one could make the reference partially opaque so there wasalways a known fraction of the ambient radiation getting through. Therewould be advantages to this in applications for gas sensors where thereference cell could be illuminated with radiation coming through thesame optical path as the sensing side except for the gas to be sensed.This would remove spurious dependencies of the signal on e.g. watervapour. A further example would be where the optical characteristics ofthe second cell are the same as that of the first cell but it isselectively illuminated with radiation of a different frequency, i.e. adifferent source of radiation, so as to provide an output which isdifferent to but which can be compared with that of the first cell. Inall cases however it will be understood that the second cell isconfigured to provide a different response output to that of the firstcell with the variance in response of this second reference cell may beprovided by altering the characteristics of the cap used for the secondcell being used to reference or calibrate the output of the first cell.

Typical embodiments will employ a reference cell with an opticallyopaque window. Such opacity may be used to provide a “dark” cell, onewhich will provide a signal output that is independent of the level ofradiation being sensed by the first cell. FIG. 8 shows an example ofsuch an arrangement. The same reference numerals will be used forcomponents already described with reference to prior Figures.

In this arrangement a sensor device 800 includes a first cell 810 whichprovides an output indicative of the level of radiation incident on thesensor device and a second cell 820 which provides an output which isindependent of the level of radiation incident on the sensor device. Thefirst and second cells each include an IR sensor 105 formed on a firstsubstrate 110 and each have a cap 816, 826 provided thereabove. Thecapping of each cell serves to define a controlled volume above eachsensor, which as described above can be suitably evacuated or filledwith a specific gas depending on the application. The second cell 820differs from the first in that it is configured so as to prevent thetransmission of radiation through the cap and onto the sensor 105. Thismay be achieved by providing an optically opaque layer 830 on the cell.The second cell can therefore be considered a reference cell, whoseoutput is independent of the incident radiation. The output of thissecond cell can then be used to calibrate the output of the first cell,whose signal output will be determined by the intensity of the incidentradiation thereon.

It will be understood that by providing such a reference cell, that asensor device in accordance with the teaching of the invention enables adetection of radiation by providing for a comparison between outputs ofan exposed sensor and those of a reference darkened sensor. In thisdevice only the optical properties of the darkened sensor are changed,the thermal and electrical properties are the same as those of theilluminated sensor. In this way an accurate and precision sensing ofincoming radiation is possible-be that IR radiation or any other type ofelectromagnetic radiation such as that in the visible spectrum.

The arrangement of the two cells shown in FIG. 8 is of two distinctcells, each being formed separately. Alternative arrangements such asthat shown in FIG. 9, may provide a single cap 900 which ismicro-machined to define two cavities or chambers 905, 910, one of which905 is locatable over the illuminated element and the second 910 overthe non-illuminated element. Each of the two defined areas has an IRsensitive element 105 and may be formed using any convenient process.The interior of the cap cavities may be filled with any desirable gasambient (e.g. Air, Nitrogen, Argon, Xenon) or indeed simply provided asa vacuum. The cap is sealed to the substrate using a sealing processwhich can provide the necessary level of hermetic seal. Such techniqueswill be apparent to the person skilled in the art. The shield 830 whichblocks the IR radiation is fabricated using (conveniently) a thin metallayer which will reflect incoming radiation. In order to avoid heatingthe cap non-uniformly, desirably the IR blocking layer should be areflector, not an absorber. As shown in FIG. 9, a gap 920 in the sealingmay be left between the individual lid chambers to allow the pressure ineach chamber to equalise, independent of leak rate of the overall cap.Such an arrangement addresses the issue with many MEMS based IR sensordevices which are very sensitive to the ambient pressure.

In order to define the two chambers, a column 925 is provided. Thecolumn extends downwardly from the top 930 of the cap 900, andterminates at the gap 920 between the two chambers. The column may becoated with or doped to minimize the leakage of radiation between thetwo columns. Typical dimensions for the column are 50-100 microns wideand 170 microns high. The gap is typically of the order of 6 micronshigh which is of the order of the wavelength of the IR radiation beingmonitored so it is unlikely that any radiation could transfer throughthe gap from the illuminated cavity to the non-illuminated. However, ifrequired further guarantees of the integrity of the dark cavity could beachieved by providing a step pattern—similar to a saw tootharrangement—so as to allow the equalisation of pressure but occlude thetransfer of radiation.

To further reduce the level of IR contamination within theun-illuminated cavity side, the walls of the separation region may alsobe coated with a reflecting metal (or other IR type barrier) to block IRwhich has been reflected from the illuminated surface. Alternativelythis region may be treated (e.g. heavily doped to sufficient densityusing for example a polysilicon material or oxidized to sufficientthickness) in such a way as to absorb any reflected IR. The absorbing ofthe radiation is a preferred way to achieve the blocking of IR throughthe internal portions of the cavity as it ensures that it is taken outof the cavity as opposed to just bounced to another region—which wouldbe the case in a reflective solution. The absorption provided by theside walls serves to damp down reflections to prevent the creation ofspurious signals within each cell A further suitable technique could beto simply space the non-illumination sensor sufficiently from theillumination sensor so that the radiation will be absorbed naturally inthe silicon.

It will be understood that a sensor arrangement in accordance with theteaching of the invention provides for the use of high thermalconductivity materials for the cap so as to ensure that the two sensingdevices are exposed to the same temperature surface, thus againminimizing thermal contamination problems. While described withreference to silicon it will be understood that other materials such asgermanium could also be used.

By using a capping arrangement such as that described herein it ispossible to locate the illuminated and non-illuminated sensors adjacentto one another. As a result they can be fabricated at the samefabrication efficiency and the only difference between the two is theoptical environment in which they operate. This is particularly usefulfor sensors that are used in high sensitivity applications where lowdifferences in output between the two sensors (the reference and theactive) are indicative of an actual measurement.

By providing at least two cells which differ in their responsecharacteristics it is possible to define such active and reference cellsas has been just described. The provision of the differing responsecharacteristics can be implemented in any one of a number of differentmanners, for example by modifying the optical response characteristics,the electrical characteristics, the thermal response characteristics oreven by keeping all these three characteristics the same and justilluminating each cell with a different source of irradiation.

Use of a Wheatstone Bridge Arrangement

While the specifics of the IR sensor (e.g. bolometer, thermopile orother) are relatively unimportant within the context of shielding, FIG.9 shows an arrangement of the IR sensor in a Wheatstone bridgeconfiguration. To function, one side of the Wheatstone bridge isrequired to be illuminated while the other side is in the dark. Usingthe structure of the cap arrangement previously described it is possibleto shield the dark side of the bridge while maintaining otherwiseidentical thermal and electrical performance of the sensing elements.Such an integrated IR sensor structure combines a highly effectivethermal management scheme, a vacuum or controlled ambient cap and amethod of providing shielding for the dark side of the bridge. The capstructure ensures that the thermal and electrical properties of theshielded and illuminated bridge elements are identical. While describedin FIG. 9 as a single device, it will be understood that such anarrangement can be applied to single sensors or arrays.

In a Wheatstone bridge configuration such as that shown in FIG. 10, aheat sensitive resistor (such as a thermistor or bolometer) on one sideof the bridge (Rbol′) is illuminated by the incoming radiation and theoutput of this side of the bridge is effectively compared to itsun-illuminated pair (Rbol) to create an output voltage which isproportional to the difference in the resistance change betweenilluminated and unilluminated resistors:Vo=VDD[(Rbol−Rbol′)/(Rbol+Rbol′)]And for Rbol′=Rbol+dRdVo˜−2dR/4Rbol

The heat sensitive resistors are characterized by having a knowntemperature coefficient of resistance (TCR), and will absorb heat fromthe incoming radiation if they are illuminated. Thus it is clear thatnot alone must the resistors (Rbol) be maintained in the dark, they mustalso see the same thermal environment as Rbol′ so that no othertemperature effects are allowed to contaminate the observed signal.While other configurations of the bridge are possible and sometimesdesirable, making the bridge from 4 identical resistors (same TCR, samethermal conductance and capacitance) in which 2 of the 4 are shieldedfrom incoming radiation while otherwise maintaining the identicalthermal environment for each of the illuminated resistors, gives optimumperformance. Using identical resistors has the effect that in theabsence of any incoming radiation the output voltage will remain zerofor any change in the background temperature of the resistors. Theresistors which are not responsive to incoming radiation are oftenreferred to as ‘reference’ bolometers.

While a radiation sensor using a Wheatstone bridge configuration usingfour physically separate resistors may provide suitable signal responsesfor certain applications, it is possible to improve the performance ofthe classic bridge configuration. An embodiment of the inventionprovides such an arrangement which improves the responsiveness of thesensor to an applied signal. In this arrangement each pair of the tworesistors that form the opposing legs of the Wheatstone bridge areco-located on a thermally isolated table so as to ensure that they eachare exposed to the same thermal environment.

As shown in the plan (FIG. 11 a) and section (FIG. 11 b) view of FIG. 11(which shows the construction of a single table with two resistors) suchan arrangement provides a thermally isolated table 1100 which is formedby etching a cavity 1105 in the silicon substrate 110. The extent of thecavity may be defined by the use of trenches 1110 which can control theextent of the etch process. The cavity serves to insulate the table 1100from the substrate below. Slots 1115 can be provided to insulate thetable from any thermal gradient in portions of the chip beside thetable. First 1120 and second 1125 resistors are provided on the table,in this illustrated exemplary embodiment in a snake (S) configuration.It will be appreciated that the actual configuration of the resistors isnot important, what is important is that the main portion of thefabricated resistor is provided on a thermally insulated table. Each ofthe two resistors are provided with contact points 1130, to facilitateconnection of the resistors to the remaining portions of the bridge.

It will be understood that in this embodiment while FIG. 11 shows theformation of a table with two bridge resistors, i.e. one pairing of theWheatstone bridge, that desirably each of the pairings of the bridgeresistors are located on their own table. In this way the formation ofthe bridge configuration will require two thermally isolated tables,each of which are desirably formed using micro-electro-mechanicalstructure (MEMS) fabrication techniques. The two resistors on oppositelegs of the Wheatstone bridge are co-located on the same table so as toensure they both see the same temperature change and, if appropriatelyconnected, provide twice the output signal for a given input radiationflux density.

The heat sensitive resistors are characterized by having a knowntemperature coefficient of resistance (TCR), and will absorb heat fromthe incoming radiation if they are illuminated by it (a suitableabsorbing layer is included in the construction of the resistors andtables). A variety of absorbing layers may be used including layers ofsilicon nitride, silicon dioxide, silver compounds and resistivecompounds such as Titanium nitride, such as are well known in thisfield. The challenge is to build the resistors in such a way that theabsorbed energy creates a sufficiently large temperature rise and thento maximize the available output signal for the given temperature rise.

There are many well known advantages of a Wheatstone bridgeconfiguration. A major benefit is that for a well matched set ofresistors (e.g. all resistors having the same TCR and value) the outputvoltage is independent of the ambient temperature and only dependent onthe total voltage across the bridge (VDD) and the localized heating ofeach illuminated resistor Rbol′. In this embodiment, the maximumpossible signal is achieved by locating the two Rbol′ units on one tableand the two Rbol units on a similar table. Their construction ensuresthat the two tables are poorly coupled thermally while ensuring theradiation sensitive resistor pairs are isothermal. Although the thermalisolation of the table is slightly degraded as for high performancedevices the thermal conductance of the table is dominated by the aspectratio of the table legs. Thus, widening the legs to accommodate tworesistors will cause a decrease in the achievable thermal resistancefrom the table to the substrate (the main heat sink in such systems). Itwill therefore be understood that as the legs affect the total DCresponse and the time constant of response of the sensor, that there isa certain trade-off possible where the designer of the system may choosedifferent dimensions of legs depending on the speed of response versusaccuracy required for the system

While the above is described in terms of a Wheatstone bridge, it is notessential that such a configuration be used. For example if the sensingresistors were to be biased by an opposing pair of current sources thesame structure could be used and would produce the same benefits. Otherapplications that would benefit from the provision of the thermallyequivalent environment generated by the location of two resistiveelements on the same thermally isolated table would include the scenariowhere a resistor was used in a feedback configuration with a secondresistor providing a sensing element—both resistors combining to providethe response of the circuit and it being important that temperaturevariances between the two resistive elements did not introduce spuriousresults.

In FIG. 11 a table with 2 legs is shown but any number of legs can beused though generally there is a trade-off between mechanical stabilityand degree of thermal isolation of the table. A symmetric arrangement oflegs is desirable for mechanical stability but is not essential.

As one side of the bridge is required to be illuminated by the incomingradiation and the other to be shielded from the radiation there will beseparation between them, enough to allow the construction of a suitableshielding structure. Such a structure could be provided by the sensorarrangement of FIG. 9 where each of the pairings is provided on athermally insulated table within a specific cavity. The IR sensors 105can be provided as a resistor bridge arrangement by fabricating thebridge using silicon micromachining technology which is sensitive toinfrared radiation. The micromachining of the two cavities 905, 910 canbe effected with one of the cavities located over the illuminatedelement (Rbol′) and the other over the unilluminated element (Rbol). Asshown in FIG. 9, the unilluminated table may have a cavity 1105underneath to give best resistor electrical and thermal matching to theilluminated side (if the incoming radiation is “sufficiently” blockedfrom reaching Rbol, or it may have no cavity underneath to provide abetter thermal short circuit (thus minimising its IR response) to thesubstrate if the incoming radiation is only partially blocked.

While this embodiment has been described with reference to a preferredimplementation where two resistive elements are provided on a thermallyisolated table it will be understood that this illustration is exemplaryof the type of benefit that may be achieved using the teaching of theinvention. Such teaching may be considered as providing at least twothermally sensitive electrical elements on a first region which isthermally isolated from the remainder of the substrate. Such thermalisolation has been described with reference to the embodiment where thetable is fabricated in the substrate, but it will be understood thatequivalently a table could be fabricated on a substrate. Such astructure could be provided by for example, depositing a sacrificiallayer on an upper surface of the substrate, then the sensor elementlayers, including support layers, and then removing the sacrificiallayer, leaving a freestanding table. Alternative implementations whereinstead of the sacrificial layer, a deposited layer is provided havinghigh thermal coefficients such that it serves to thermally isolate theformed sensor elements located thereabove from thermal effects presentin the substrate. These and other modifications will be apparent to theperson skilled in the art as a means to provide a thermal barrier underthe electrical elements where high degrees of thermal isolation arerequired.

Thermal barrier for thermally isolating portions of the die

As will be understood from the preceding, thermal sensors and otherelectrical elements can be effected by the temperature of the supportingsubstrate. It will be understood that thermal sensors specifically aresensitive by design to changes in temperature and often use thetemperature of the supporting substrate as a reference or baselinetemperature. However, if the sensor incorporates heat generating means(e.g. circuits) which are nearby then this reference or baselinetemperature will be disturbed and will give rise to an error in thecalculated temperature measured by the sensor.

FIG. 12 shows another embodiment which illustrates a means of creating athermal barrier between any heat generating region and the thermalsensor to isolate the sensor from any spurious source of heat. Thedegree of thermal isolation required can be engineered as needed. Thisembodiment uses pairs (or more, e.g. triples, etc.) of trenches spacedby gas or vacuum containing regions coupled possibly withsilicon-on-insulator wafers to provide a high degree of thermalisolation.

While important for all such sensors and circuits the problem becomesparticularly acute if the sensor is attempting to measure a thermalsignal itself, e.g. an infrared sensor or a micro-calorimetry sensor,and therefore is particularly suited for incorporation in the sensorarrangements described with reference to FIGS. 1 to 11. Such sensorsgenerally take the substrate temperature as a reference or baselinetemperature and perform a comparison of their internal temperature withthe substrate reference or baseline temperature. Any offset or unequaldistribution in the baseline temperature, whether steady state or timevarying can lead to inaccuracies in the sensor response in essentiallydirect proportion to the experienced offset.

Any nearby circuitry which is dissipating heat will cause a localtemperature rise around the power dissipating element. This heat isconducted away from the element in a manner which is well controlled andunderstood according to the thermal conductivity properties of thematerials used. While the thermal barrier may be suited for applicationwith the capped sensors heretofore described, it will be now illustratedwithout reference to such capping.

This embodiment of the invention provides for a means for increasing thethermal resistance between a heat source and a sensor or sensitivecircuit to reduce the impact of this extraneous heat source. Withreference to FIG. 12, the main features of the structure and a method ofmaking it are described. The same reference numerals will be used forparts previously described with reference to any one of FIGS. 1 to 11. Asilicon or other suitable wafer is used as the substrate 110. The sensoris formed by any suitable method (the order of fabrication is notimportant) and then two adjacent deep trenches 1205, 1210 are etched inthe substrate so as to form a ring around either the sensor 105 or theheat dissipating source 1215 and filled with an appropriate fillingmaterial 1205 a, 1205 b (e.g. silicon dioxide, silicon nitride,polysilicon or any combination of these). Each of the two trenchesthereby form a first and second region having a first thermalcoefficient and are separated from one another by an intermediary region1220 which has a second thermal coefficient. This intermediary regionmay conveniently be provided by etching a region between the twotrenches to form an air filled region which may be capped by means of aninsulating cap 1225 to form an evacuated air filled region. Of coursethe available volume within this region 1220 could alternatively befilled with other gaseous compositions, as required. The remainder ofthe circuit elements are formed by any of the usual integrated circuitmethods leaving the insulating cap 1225 over the trenches and the regionbetween them. Occasional holes 1230 for etchant ingress are formed inthis insulating layer exposing the silicon between the adjacenttrenches. The entire wafer is then exposed to an etching medium, (e.g.XeF2 if a gas or KOH if a liquid etchant), chosen so as to etch only thesilicon, which removes the silicon between the trenches. The low thermalconductivity of this gas filled space ensures that little heat istransferred from source to sensitive element down to the depth of thetrenches and removed zone. The heat is forced to flow under the trenchedand etched area thus increasing the effective thermal resistance betweenthe heat source and the sensitive element.

Increased levels of thermal isolation can be obtained in a number ofdifferent manners such as by (1) using multiple such thermal barriers,(2) increasing the depth of the trenches and removed zone relative tothe overall depth of the silicon substrate, (3) as shown in FIG. 12 c,by extending the etch sufficiently that an undercutting 1240 under thesensor element is defined so creating to a greater or lesser extent anenclosed isolated “island” 1250 or (4) by using a substrate ofsilicon-on-insulator material 1301 and ensuring that the trenches areetched to the depth of the buried insulator layer as shown in FIG. 13.The person skilled in the art will be familiar in the fabrication ofstructures incorporating such buried oxide layers. As the oxide or otherdielectric layers have much lower thermal conductivity thansemiconductor or metallic layers, the use of such materials mayadditionally improve the thermal isolation achieved. The extent of theetch shown in FIG. 12 c can be controlled by having one of the trenchesformed deeper than the other, typically the outer trench, so that theetch is achieved preferentially in a direction under the sensor. Theprovision of this evacuated region under the sensor creates a region ofdifferent thermal characteristics to that of the surrounding siliconsubstrate and thereby provides increased thermal insulation for thesensor located above.

Advantageously the trenches could be filled with a dielectric such assilicon dioxide provided using any known method. However, because of thedifference in coefficients of thermal expansion between silicon dioxideand silicon it has been more common to use a thin layer (e.g. 100-200nm) of silicon dioxide or silicon nitride or both to line the etchedtrenches and then fill the bulk of the trench with polysilicon. Thissignificantly decreases the effectiveness of the trench as a thermalbarrier due to the high thermal conductivity of polycrystalline silicon.In principle the trenches could be left unfilled after the silicondioxide layer is deposited but this gives rise to problems of surfacepotential control at the trench/silicon substrate interface which isgenerally undesirable if the trench has any function other than thermalisolation. Also, as it is desired to ensure cost is kept to a minimum,our approach uses steps which are in many situations used for thefabrication of the sensor itself and thus add no further cost to thefabrication process. Due to the surface potential control problemmentioned above, this would probably not be acceptable in the case foran unfilled trench (i.e. a trench used in the process for electricalisolation could not tolerate such a surface potential control issue). Itis also frequently desired to perform the trench processing at thebeginning of the fabrication sequence for the whole process and anunfilled trench could not be tolerated in this case as it would fillwith process residues leading to unmanageable defect levels due to itsopen top. The method we suggest is intended to be carried out at the endof the process sequence, using trenches which have been fabricated atany point in the process, thus avoiding any such issues.

The formation of a thermal barrier, by defining regions of differentthermal coefficients, around the temperature sensitive element serves toisolate the element from the effects of any heating from adjacentcomponents. It will be appreciated that it may still be necessary toelectrically couple components within the thermal barrier region tothose outside the region. Such coupling may be achieved by providing anyone of a number of different types of electrical connection, such as awire track 1235, between the components that need to be coupled.Depending on the circumstances of application of such sensors differentdegrees of thermal isolation may be required which will affect theultimate thermal barrier configuration chosen.

Distributed Die Temperature Sensor

While the sensors heretofore described have been described withreference to stand-alone sensors or arrays of such sensors, in anotherembodiment of the invention an arrangement which provides for dietemperature sensing is also provided. Such an arrangement is shown inFIGS. 14 to 17.

The provision of such die sensing provides a means of measuring the dietemperature at a number of locations around the die, these temperaturemeasurements can then be used to re-compensate the apparent observedtemperature. This may be done by locating small temperature measurementmeans, such as sense diodes and/or transistors at strategic pointsaround the die and using temperature sensor circuits to measure thesespot temperatures, providing the data to the user. Where used incombination with the thermal barrier that was illustrated above withreference to FIGS. 12 & 13, the efficacy of the thermal barrier can alsobe checked in production by applying a known heat source and measuringthe change in temperature across the barrier, dT, using sensors locatedon either side of the thermal barrier. It is known that any circuitrywhich is dissipating heat will cause a local temperature rise around apower dissipating element. This heat is conducted away from the elementin a manner which is well controlled and understood according to thethermal conductivity properties of the materials used. In addition,other die temperature changes at local or wider levels can be caused byexternal sources of radiation or ambient temperature changes. Forexample, if an IR thermal sensor is measuring a scene which includes amoving hot-cold edge (e.g. a hot object on a conveyor), a dietemperature change will occur on the sensor die, moving from one edge tothe other as the object transits the field of view. Likewise, if thesame thermal sensor is carried from a cold environment to a hot one, thebase die temperature will change with some appreciable time lag causingreading inaccuracies. Thus, both global die temperature changes andfixed or time varying temperature gradients can cause severe measurementinaccuracies.

Another problem that occurs for MEMS implementations of the thermalsensor, where a thermal barrier has been etched into the siliconsubstrate, is that the thermal barrier may be breached or incompletelyformed, either at the silicon processing stage or during the cappingprocess that was described with reference to previous figures. Somemeans of checking this barrier at probe and final test is useful toeliminate poorly performing die.

The problem can be significantly better managed if a number oftemperature sensors are located around the die area and the localisedtemperature readings are then used to compensate the IR thermal sensormeasurements. The application is particularly important for thermalsensor arrays used for radiometric applications (i.e. where actualtemperature measurement is needed as opposed to thermal imaging), wheredie temperature gradients can cause severe temperature measurementinaccuracies.

In this embodiment of the invention, we disclose within a system such asa thermal infrared or microcalorimetric sensor or imaging system, adistributed set of temperature sensing points located within and withoutthe thermal barrier (i.e. on either side of a thermal barrier), if thatexists. These temperature sensing points can be made in many ways butadvantageously they are made using PN junctions which are then drivenwith circuits well known to those skilled in the art. FIG. 14 shows howsuch an arrangement might look. The same reference numerals are used forcomponents described with reference to previous drawings.

As shown in FIG. 14 a plurality of individual die temperature sensors1400 are provided, and are arranged about the die. In the views of FIG.14, a plurality of individual IR sensors 105, similar to those describedpreviously, are provided within a thermal barrier region 1410 defined bytrenches 1205, 1210. Die temperature sensors 1400 are provided withinthis thermal barrier region to monitor the die temperature within theregion. Additional die temperature sensors are provided outside thethermal barrier region 1410. To check the thermal barrier integrityformed by the trench arrangement, these temperature sensing devices 1400are combined with sources of known heat load such as resistors 1215. Insome embodiments the heat load function can be combined with a known TCRresistor in a single unit thus combining the functions of heating andtemperature sensing to minimise space use. Alternatively, the heatercould also simply be a known source of heating on the die e.g. a nearbyhigh current region such as the input stage of an amplifier. The arraytemperature sensors could also be used to perform the same function,i.e. for a known circuit condition where a source of heat is availableexternal to the thermal isolation barrier, the internal array of sensorswill show a specific pattern of temperature measurements. Any deviationfrom this once characterised, will point out a thermal barrier defect.

When a known heat load is applied through the resistor(s) temperaturescan be measured on either side of the barrier to ensure its integrity.These temperature differences would be characteristic of any givensystem and the package it is located within. Any defect or fault in thebarrier (e.g. bridging of the thermal isolation trench by someextraneous material) would result in a temperature difference smallerthan expected.

To assist in improving the accuracy of the thermal sensor measurement,the die temperature measurement devices 1400 need to be distributedaround the die within the thermal barrier so that local, time varyingtemperature measurements of the die next to individual sensor pixels andgradients across the die can be known. The user can then select to makeeither average die temperature measurements (average all readings)during the course of a measurement or in applications which experiencesharp thermal scene temperature differences (either spatially ortemporally) the local temperature reading can be used to improve thetemperature measurement accuracy of any individual pixel.

FIGS. 15 to 17 show a distributed temperature sensor for a small array.A variety of temperature sensor placing strategies can be used. As shownin FIG. 15, temperature sensors 1400 may be placed at all corners of thesmall 3×3 pixel array 1500. This gives detailed knowledge of the dietemperature “map” but is costly in terms of area used and wiringrequired to connect all the sensors. FIG. 16 shows an alternative where,for this size array, all outside corners and all 4 interior corners hosttemperature sensors 1400. This gives 2 sensors for each pixel and 4 forthe centre pixel, which could be advantageous in some applications wheregreatest accuracy is required of a specific pixel in the array. FIG. 17shows a minimal coverage situation where only the 4 interior cornershost temperature sensors. In this situation there is at least onetemperature sensor per pixel, some have two and the centre pixel againhas 4 sensors. This distribution may be less effective in the detectionof thermal barrier defects (depending on the size of the array and/orarea inside the thermal barrier) as the sensors are located further fromthe barriers. It will be understood from the different examples of FIGS.15 to 17 that different applications will require other distributions ofdie temperature sensors. Furthermore it will be understood that thedistribution of the individual die temperature sensors may require someof the die temperature sensors to be located under the caps of thesensor with others located outside the caps.

It will be understood that an arrangement such as that provided by thepresent invention offers many advantages over the existing state of theart. Current practice in thermal radiometric measurement systems is tomeasure the die temperature with an external temperature sensor locatedin close thermal contact or proximity with the sensor package. Thisambient temperature measurement unit is usually mounted on the same PCBor may be mounted in physical contact with the radiometric sensor orarray. Some sensors and arrays will have a sensor located physically onthe same die but never an array of die temperature sensors has beenused. If, the die temperature sensors are made using known circuittechniques for building active temperature sensors then the user willhave access to pre-calibrated die temperature information in greatdetail, not requiring him to undertake extensive calibration of thisparameter.

Sensing the die temperature with an additional sensor located outsidethe package gives rise to an apparent thermal lag between actual arraytemperature and ambient or PCB temperature. This leads to unacceptablylong periods of inaccurate readings for many applications. The schemedisclosed here provides far superior measurement of the die temperaturewithout this thermal lag.

It will be understood that the sensors described herein have beenillustrated with reference to exemplary embodiments. It will beunderstood that the features of any one embodiment may be used withthose of another embodiment or indeed can be applied independently ofthe structural features of the other embodiment. Applications for suchsensors can be in a plurality of environments such as IR to Digitalconverters, both single pixel and arrays. Further applications includesingle point thermal measurement systems, e.g., digital thermometers,intruder alarms, people counting sensors, and into infra-red cameras tothermally image scenes. These and other applications will be readilyapparent to the person skilled in the art on review of the teaching setforth herebefore. Therefore while the invention has been described withreference to preferred embodiments it will be understood that it is notintended that the invention be limited in any fashion except as may bedeemed necessary in the light of the appended claims.

The words upper, lower, inner and outer are used for ease of explanationso as to illustrate an exemplary illustrative embodiment and it in notintended to limit the invention to any one orientation. Similarly, thewords comprises/comprising when used in this specification are tospecify the presence of stated features, integers, steps or componentsbut does not preclude the presence or addition of one or more otherfeatures, integers, steps, components or groups thereof. Furthermorealthough the invention has been described with reference to specificexamples it is not intended to limit the invention in any way except asmay be deemed necessary in the light of the appended claims, and manymodifications and variations to that described may be made withoutdeparting from the spirit and scope of the invention.

1. An electrical circuit fabricated on a semiconductor substrate, thecircuit including first and second thermally sensitive electricalelements each providing an output which contributes to the overalloutput of the circuit and wherein the electrical elements are co-locatedon first region which is thermally isolated from the first substrate. 2.The circuit as claimed in claim 1 wherein the first region is formed inthe first substrate.
 3. The circuit as claimed in claim 1 wherein thefirst region is formed on the first substrate.
 4. The circuit as claimedin claim 1 wherein the first region is thermally isolated from the firstsubstrate by providing an evacuated cavity below the first region. 5.The circuit as claimed in claim 1 wherein the first region is thermallyisolated from the first substrate by providing an insulating layerbetween the first region and the substrate below.
 6. The circuit asclaimed in claim 1 wherein the first region is suspended relative to thesubstrate.
 7. The circuit as claimed in claim 1 wherein the first regionprovides a substantially isothermal structure.
 8. The circuit as claimedin claim 1 wherein the first region is coupled at one or more edgeportions to the substrate.
 9. The circuit as claimed in claim 8 whereinthe first region is provided as a table, the electrical elements beingformed on an upper surface thereof, the table being supported relativeto the substrate by the provision of one or more legs, the leg(s) beingprovided at edge portions of the table.
 10. The circuit as claimed inclaim 1 wherein the electrical elements are resistive elements.
 11. Thecircuit as claimed in claim 1 configured as a thermal sensor, each ofthe first and second electrical elements providing an output indicativeof the temperature sensed by that element.
 12. The circuit as claimed inclaim 11 wherein the sensor is a thermal radiation sensor with each ofthe first and second electrical elements provide an output indicative ofthe radiation incident on that element.
 13. The circuit as claimed inclaim 1 further including third and fourth thermally sensitiveelectrical elements, the third and fourth elements being co-located on asecond thermally isolated region.
 14. The circuit as claimed in claim 13wherein the electrical elements are arranged in a bridge configuration.15. The circuit as claimed in claim 14 wherein individual legs of thebridge configuration are provided by each of the first, second, thirdand fourth electrical elements.
 16. The circuit as claimed in claim 15wherein each of the elements on each thermally isolated region providecomplementary outputs to the other of the elements on the respectivestructure.
 17. A thermal sensor including a circuit as claimed inclaim
 1. 18. A thermal sensor fabricated on a semiconductor substrate,the sensor including: a) a first temperature sensing element havingfirst and second radiation elements and providing an output whichcontributes to the overall output of the thermal sensor and b) a secondtemperature sensing element having first and second radiation elementsand providing an output which contributes to the overall output of thethermal sensor, and wherein the first and second temperature sensingelements are located on first and second regions, the first and secondregions being thermally isolated from the first substrate.
 19. Thesensor of claim 18 further including a first and second cap formed in asecond substrate, the first and second substrates being arrangedrelative to one another such that each of the first and secondtemperature sensing elements having a respective cap providedthereabove, and wherein the cap for the first sensing element allows atransmission of incident radiation through the cap and onto the sensingelement and the cap for the second sensing element blocks at least aportion of radiation being transmitted through the cap and onto thesecond sensing element.
 20. The sensor of claim 19 wherein the cap forthe first sensing element includes an optical element configured toprovide a focusing of the incident radiation onto the first sensingelement.
 21. The sensor of claim 19 wherein the cap for the secondsensing element includes a reflective coating which reflects radiationincident on the cap.
 22. The sensor of claim 19 wherein the cap for thesecond sensing element includes an optically opaque coating so as toprevent transmission of radiation through the cap and onto the secondsensing element.
 23. The sensor as claimed in claim 19 wherein thearrangement of the first and second substrates relative to one anotherdefine a cavity between each of the caps and their respective sensingelements.
 24. The sensor as claimed in claim 23 wherein each of thecavities for the first and second sensing elements are in fluidcommunication with one another.
 25. The sensor as claimed in claim 23wherein each of the cavities for the first and second sensing elementsare isolated from the other of the cavities for the first and secondsensing element.
 26. The sensor as claimed in claim 19 wherein the firstand second substrates are provided in silicon.
 27. The sensor as claimedin claim 18 wherein the first and second sensing elements are infra-redsensing elements.
 28. The sensor as claimed in claim 20 wherein the atleast one optical element is a diffractive optical element.
 29. Thesensor as claimed in claim 20 wherein the at least one optical elementis a refractive optical element.
 30. The sensor as claimed in claim 23wherein the ambient conditions and composition with the cavities can bespecified.
 31. The sensor as claimed in claim 30 wherein the cavitiesare provided at a pressure lower than ambient pressure.
 32. The sensoras claimed in claim 30 wherein the cavities are populated with a gaseouscomposition selected for the application with which the sensor is to beused.
 33. The sensor as claimed in claim 32 wherein the gaseouscomposition comprises a gas having a thermal conduction less than thethermal conduction of nitrogen.
 34. The sensor as claimed in claim 20wherein the optical element is formed in an inner surface of the cap,adjacent to a chamber formed above the first sensing element.
 35. Thesensor as claimed in claim 20 wherein the optical element is formed inan outer surface of the cap, remote from a chamber formed above thefirst sensing element.
 36. The sensor as claimed in claim 20 whereinoptical elements are formed in both an outer surface and inner surfaceof the cap for the first sensing element, the combination of the opticalelements forming a compound lens.
 37. The sensor as claimed in claim 20wherein a plurality of first sensing elements are formed and the opticalelement is configured to selectively guide radiation of specificwavelengths to preselected ones of the plurality of first sensingelements.
 38. The sensor as claimed in claim 19 wherein the caps for thefirst and second sensing element are formed in the same secondsubstrate, the sensor additionally comprising an outer cap orientatedover the second substrate, the cap including an optical element.
 39. Thesensor as claimed in claim 19 wherein on arranging each of the first andsecond substrates relative to one another each of the caps are formed byside walls extending upwardly from the first substrate and supporting aroof therebetween, the roof being in a plane substantially parallel tothe sensing elements.
 40. The sensor as claimed in claim 39 wherein eachof the first and second sensing elements are adjacent to one another,the caps provided thereabove sharing a common central column, thatextends downwardly from the roof, thereby defining chambers for each ofthe first and second sensing elements.
 41. The sensor as claimed inclaim 40 wherein the chamber for the second sensing element is treatedto prevent a transmission of radiation through the cap and onto thesecond sensor element.
 42. The sensor as claimed in claim 41 wherein thetreatment includes a doping of the side walls of the chamber.
 43. Thesensor as claimed in claim 41 wherein the treatment includes theapplication of a reflective coating on the roof of the cap for thesecond sensing element.
 44. The sensor as claimed in claim 40 whereinthe central column does not extend fully from the roof to the firstsubstrate, such that a gap is defined between a lower surface of thecolumn and an upper surface of the first substrate.
 45. The sensor asclaimed in claim 44 wherein the width of the gap is comparable with thewavelength of the incident radiation being sensed.
 46. The sensor asclaimed in claim 44 wherein the provision of the gap allows for anequalisation of pressure between the chambers for the first and secondsensor element.
 47. The sensor as claimed in claim 18 wherein each ofthe first and second sensing elements are provided as a bolometer. 48.The sensor as claimed in claim 18 wherein at least a portion of each ofthe first and second sensing elements are suspended over a cavitydefined in the first substrate, the extent of the suspension defining athermally insulated table forming the first and second regions, thecavity providing thermal insulation between the sensing elements and thesubstrate.
 49. The sensor as claimed in claim 18 wherein the first andsecond sensing elements are arranged in a Wheatstone bridgeconfiguration.
 50. The sensor as claimed in claim 49 wherein theWheatstone bridge configuration is provided by the first sensing elementhaving a first pair of resistive elements and the second sensing elementhaving a second pair of resistive elements, resistors from each pairdefining opposite legs of the Wheatstone bridge.
 51. The sensor asclaimed in claim 50 wherein each of the resistors on opposite legs ofthe Wheatstone bridge are co-located on a dedicated thermally isolatedtable.
 52. The sensor as claimed in claim 51 wherein the thermallyisolated table is fabricated using micro-electro-mechanical techniques.53. The sensor as claimed in claim 18 wherein the first substrateincludes a trench arrangement located outside the sensor, the trencharrangement providing thermal insulation between the sensor and otherelements on the first substrate.
 54. The sensor as claimed in claim 53wherein the trench arrangement includes two adjacent trenches etched inthe first substrate, each of the two trenches being filled with athermally insulating material and being separated from one another by anintermediary region having a thermal coefficient different to that ofthe trenches.
 55. The sensor as claimed in claim 54 wherein the trencharrangement is located around the sensor so as to define a thermalbarrier around the sensor.
 56. The sensor as claimed in claim 55 whereinthe trench arrangement is located between the sensor and a heat sourceprovided on the first substrate.
 57. The sensor as claimed in claim 53wherein the trench arrangement includes a plurality of trenches, theplurality of trenches being provided in pairs of adjacent trenches beingseparated from one another by a cavity.
 58. The sensor as claimed inclaim 53 wherein the first substrate includes a buriedsilicon-on-insulator layer and the depth of the trenches forming thetrench arrangement is such as to extend to the buried layer.
 59. Thesensor as claimed in claim 57 wherein the cavity extends below thesensor.
 60. The sensor as claimed in claim 18 further including aplurality of die temperature sensors, the plurality of the dietemperature sensors providing an output indicative of the temperature ofthe die on which the sensor is located.
 61. The sensor as claimed inclaim 60 wherein the plurality of die sensors are arranged about thedie, so as to give a plurality of output measurements, each of theoutput measurements being related to the temperature at the location ofthat die temperature sensor.
 62. A sensor array including a plurality ofsensors, each of the sensors having an active sensor element and areference sensor element, the active sensor element being formed on athermally isolated table in a first substrate and having an opticalelement formed in a second substrate, the first and second substratesbeing configured relative to one another such that the second substrateforms a cap over the sensor element, the optical element beingconfigured to guide incident radiation on the cap to the sensingelement, the reference sensor element also being formed in thermallyisolated table in the first substrate and having a cap formed in thesecond substrate, the first and second substrates being configuredrelative to one another such that the cap is located over the referencesensor element, the cap serving to shield the reference sensor elementfrom at least a portion of incident radiation on the cap, the active andreference sensor elements being arranged in a bridge configuration. 63.The sensor array of claim 62 wherein an output of the sensor arraydefines an image plane.
 64. A discriminatory sensor configured toprovide a signal on sensing a heat emitting body, the sensor including afirst sensor element configured to provide a signal on sensing the bodya first distance from the sensor and a second sensor element configuredto provide a signal on sensing the object a second distance from thesensor, each of the first and second sensor elements including at leastone sensing element located on a thermally insulated table formed on afirst substrate and at least one optical element formed in a secondsubstrate, the first and second substrates being configured relative toone another such that the second substrate forms a cap over the at leastone sensing element, the at least one optical element being configuredto guide incident radiation on the cap to the at least one sensingelement.
 65. The discriminatory sensor of claim 64 wherein the object isa human torso.
 66. A gas analyser including at least one sensor elementformed in a first substrate and at least one optical element formed in asecond substrate, the first and second substrates being configuredrelative to one another such that the second substrate forms a cap overthe at least one sensor element, the at least one optical element beingconfigured to guide incident radiation on the cap to the at least onesensor element, the incident radiation guided having a wavelengthindicative of the presence of a specific gas, the gas analyser includingat least one reference sensor element formed in the first substrate andhaving a cap for the at least one reference sensor element formed in asecond substrate, the cap serving to shield the reference sensor elementfrom at least a portion of the incident radiation on the cap such thatthe reference sensor element provides an output independent of theintensity of the incident radiation, the reference sensor element andsensor element each being formed on individual thermally isolated tablesdefined in the first substrate and being arranged collectively in abridge configuration.
 67. The gas analyser of claim 66 including aplurality of sensor elements and a plurality of associated opticalelements therefore, each of the combined sensor elements and opticalelements being configured for specific wavelength analysis such that theoutput of the plurality of sensor elements may be used to provide a gaswavelength signature spectrum.
 68. A method of forming a sensor, themethod including the steps of: forming at least one sensor element andat least one reference sensor element in a first substrate, etching acavity below a portion of each of the sensor elements to thermallyisolate the sensor elements from the substrate below, forming an opticalelement and a shielding cap in a second substrate, bonding the first andsecond substrates together such that the second substrate provides theoptical element over the sensor element, the optical element isconfigured to guide incident radiation onto the sensor element and thesecond substrate provides the shielding cap over the reference sensor,the shielding cap serving to prevent a transmission of incidentradiation on the cap onto the reference sensor element.
 69. The methodof claim 68 wherein the sensor elements are arranged in a bridgeconfiguration.
 70. An electromagnetic sensor fabricated in asemiconductor process, the sensor including first and second sensingelements formed in a first substrate, each of the first and secondsubstrates having a respective cap defined thereabove, the caps beingformed in a second substrate and being mountable onto the firstsubstrate and wherein the cap formed over the first sensing elementallows a transmission of radiation through the cap onto the sensingelement and the cap formed over the second sensing element filters thetransmission of at least a portion of the radiation through the cap ontothe sensing element so as to reduce the radiation incident onto thesecond sensing element relative to that incident onto the first sensingelement, each of the first and second sensing elements being provided ona thermally isolated table on the first substrate and being electricallycoupled to one another in a bridge configuration.